This invention relates to a transmission type screen unit that displays an image while maintaining a flatness, and a projection type display apparatus using the screen unit.
Recently, a projection type display apparatus is broadly used as a large-sized display device for displaying an image information. The projection type display apparatus is constructed to project an image on a transmission type screen, and the size of the projection type display apparatus tends to be large. As the transmission type screen, the projection type display apparatus employs a screen (hereinafter referred to as a screen panel) including a plurality of sheet shaped lenses (hereinafter, referred to as lens sheets) such as a Fresnel lens and a lenticular lens laminated with a glass plate.
In the above constructed screen panel, the image quality is maintained by keeping the incident angle of the incident light on the screen panel to a designed angle, and therefore it is important to maintain the flatness of the screen panel. Further, in order to ensure the focusing performance, it is necessary that the laminated lens sheets are attached to each other as closely as possible. Therefore, the screen panel is so constructed that the flatness of the screen panel is maintained and the laminated lens sheets are closely attached to each other.
However, the sheet shaped Fresnel lens and the lenticular lens used in the screen panel are made of resin material having the thickness of several millimeters, in order to reduce the weight of the apparatus and to ensure the optical transmission. Therefore, the lens sheets may expand, contract or become warped according to the surrounding condition (i.e., temperature or humidity) or an externally applied stress, with the result that the image quality may be degraded. In order to prevent the problem, it is proposed to previously warp the lens sheets before the lens sheets are closely attached to each other (for example, Patent document 1). Further, it is proposed to closely attach the lens sheets to the glass plate having a rigidity (for example, Patent Document 2).
However, according to the upsizing of the screen unit, the areas of the lens sheets tend to be large, and therefore it becomes difficult to sufficiently restrict the expansion, contraction or warping of the lens sheets, even if the lens sheets are closely attached to the rigid glass plate or the like. Moreover, if the expansion, contraction and warping of the lens sheets are to be restricted by increasing the thickness of the lens sheets, the cost may increase. Further, there is a difference in hygroscopic property between a surface of the lens sheets contacting the glass plate and the opposite surface of the lens sheets exposed to the outside air, and therefore the separation of contact surfaces of the lens sheets may occur, so that the lens sheets may be warped.
In order to solve these problem, it is proposed to evacuate the air between the laminated lens sheets to reduce the pressure to be lower than the atmospheric pressure so that the lens sheets are closely attached to each other (for example, Patent Documents 3 and 4).
[Patent Document 1]
    Japanese Laid-open Patent Publication No. 2001-356419 (see FIG. 1)[Patent Document 2]    Japanese Laid-open Patent Publication No. 2001-154274 (see FIG. 3)[Patent Document 3]    Japanese Laid-open Patent Publication No. 06-194740 (see FIG. 1)[Patent Document 4]    Japanese Laid-open Patent Publication No. 08-314015 (see FIG. 2)
However, in the method in which the air between the lens sheets is evacuated while the screen is manufactured (Patent Document 3), the air may flow into between the lens sheets because of minute imperfections of the sealing member or the deterioration of the sealing member with age. Further, in the method in which the air between the lens sheets is evacuated while the screen is used (Patent Document 4), the air may not be sufficiently evacuated throughout the whole surfaces of the lens sheets if the lens sheets become large. Particularly, in a part far from the evacuating portion (such as an evacuating fan), the pressure between the lens sheets are not sufficiently reduced, and therefore the flatness of the screen panel may not be maintained. In addition, the lens sheets may be warped because of the humidity, so that the image quality may be degraded.